For me, there are two: the version of “Gaslight” with Ingrid Bergman, and a “Christian” movie that came out in the early 1970s called “A Thief In The Night”.
I started a thread about this on another website a couple years ago, and the most unexpected reply was “All The President’s Men”.
Martyrs. Hands down one of the best horror movies of the past decade and, in my opinion, ever. Words do it no justice. Warning, though: Not for the faint of heart.
The Tell-Tale Heart. I saw it in elementary school–I think it was a cartoon.
Idiocracy, you can see it happening as we speak…
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Fail-Safe. It was all too possible at the time. We don’t talk about tactical nukes anymore, but that doesn’t mean they’re not still there.
Movies don’t really scare me, but ‘The Exorcist’ came the closest when I first saw it as a teenager.
I thought The Descent was tense from the start. Then it really went to hell.
The Exorcist for me as well. Couldn’t sleep after I saw that movie.
Silence of the Lambs. Buffalo Bill was just real enough to be frightening, and then when we came home from the theater, there was a light on we didn’t remember leaving on.
The Mothman Prophecies. For some damn reason, the mothman was the bogeyman of my childhood. There were a couple of contenders, but those were more of the monsterish variety, perhaps most importantly, entities with some clear definition and motives—vampires wanted to suck your blood, mummies were reanimated Egyptian kings, things that, even in their alienness, ultimately had some sort of a comprehensible nature. But the mothman, I think, was my first brush with Lovecraftean ‘cosmic horror’ that is not so much malignant, but merely impossibly other, utterly different. I don’t even know why, perhaps it was just the inherent ridiculousness of a ‘moth-man’ that hinted at a deeper and incredibly alien nature of which we can only perceive the merest glimpse.
The film, which came much later, captures that aspect sufficiently well to make chills run down my spine even today. (And in some strange way, I must admit, this horror has become something I cherish.)
I have The Changeling as my go-to scary movie. Not gory or shocking, but a deeply creepy and unsettling old-fashioned ghost story.
Saw it when I was a young teen, and certain scenes gave me the horrors for weeks afterwards.
I suppose*** Jaws and Rear Window ***were the movies that scared me the most when I first saw them.
Unless you count the flying monkeys in The Wizard of Oz- granted, I was only 3 when I first saw them, but they terrified me.
You didn’t grow up in the Point Pleasant area, did you?
As for me… hmmm, I’d almost say “Black Sunday” (US title for “Mask of the Demon”) starring Barbara Steele as a witch/vampire returned centuries after being burned & having a mask nailed to her face back centuries ago.
“A Thief In the Night” has cheap production values but more soul than any of the Left Behind films. ATITN & its three sequel were cobbled together by people who put their all into them because, by God, they believed the message & wanted to deliver a gut punch.
My parents offered to take me with them when they went to see Jaws in the theater. Somehow, even at 6 or 7, I knew to decline.
Heh, I saw that movie when I was (checks) 9. My dad was working at the time at the Marine Biological Laboratories in - Wood’s Hole, Massachusetts. 
Made swimming lessons that summer … entertaining.
Yessir. This movie was scary even before the CHUDS showed up.
Truly one of the scariest movies I’ve ever seen.
No, but I had (still have, really) a huge fascination with all things scary, creepy, and unexplained, so I had this series of (ultimately really crappy) booklets about ‘unexplained phenomena’ which featured the mothman, among the more routine creeps like el chupacabra, the Jersey devil, aliens, bigfoot etc. Mothman somehow hit a chord, I guess.
For me, it was The Shining.
I saw Phantasm as a sophomore in college.
That was the only movie, to this point, that has caused me to have nightmares.
I think it was the movie score.
And the silver ball of doom.
A lot of movies scared me in the movies but only one TV movie (mini-series, really) scared the dickens out of me as an adult. Salem’s Lot. I do not remember details-- except for the child hovering at the second floor window asking his bestest friend to please let him in so they could play. Even though I know the whole mini-series spooked me, this scene still haunts me.